The 26-year-old has long been linked with a move to the La Liga champions, although an official agreement between the two sides has yet to be reached.
"You read many things in the media but from the information that I have, Luka will move to Real Madrid for sure," Mamic told 24sata.
"I've heard it's almost done, so I don't have any doubts Luka will play for Real Madrid next season. It is just a question of when the official announcement will be."
Modric has not featured for Spurs this season.